The spectrum subtraction method is one of various techniques for suppressing noise that is included in a sound. The spectrum subtraction method determines a spectrum of an observation signal in which noise is superimposed on a sound (hereinafter referred to as “observation signal spectrum”), estimates a spectrum of noise (hereinafter referred to as “noise spectrum”) from the observation signal spectrum, and obtains a spectrum of a noise-suppressed sound (hereinafter referred to as “sound spectrum”) by subtracting the noise spectrum from the observation signal spectrum. The spectrum subtraction method then produces a noise-suppressed sound by converting the sound spectrum into a signal in the time domain.
Examples of conventional techniques that include the spectrum subtraction technique are described in the following patent documents:    [Patent document 1] JP-A-11-3094    [Patent document 2] JP-A-2002-14694    [Patent document 3] JP-A-2003-223186
In the conventional spectrum subtraction method, a common observation signal spectrum is used as an observation signal spectrum used for estimation-calculating a noise spectrum (hereinafter referred to as “noise estimation spectrum”) and as an observation signal spectrum as a minuend from which to subtract the noise spectrum (hereinafter referred to as “noise suppression spectrum”).